September 15, 2007

Sep. 15--Q:I grow tomatoes every year and find those big green worms on them. Some have white lumps on their backs. I've heard they are some kind of eggs. Is that true? If so, are they harmful?

Answer: The worms you're seeing are tomato hornworms. They are common and enjoy eating the leaves of tomato plants. The white lumps are the egg cases for a parasitic wasp. This wasp stings the worm and deposits its eggs on the worm's back. As the young wasps develop, the egg cases enlarge to the stage that you notice them. At the same time, the developing wasps are slowly draining the life from their host hornworm.

Are these wasps harmful? No, in fact, they are considered beneficial because they eventually kill the worm that is eating the leaves of your tomato plants. I always tell home gardeners who see a hornworm carrying white egg cases to protect it and don't let anyone kill the worm before its time. When these parasitic wasps emerge, you'll have even more beneficial insects working around your garden.

. . .

Q:It's only early September, but the leaves on my tulip poplar have been turning yellow and dropping for the last three or four weeks. Are these trees dying from this summer's drought? Is there anything I can do to protect them?

Answer: The tulip poplars in my neighborhood have been doing the same thing. It is a reaction to the drought and humidity, but they are not dying.

When we sweat, it's called perspiration. When trees sweat, it's called transpiration. In the case of the poplar, these trees weren't able to take in enough water through their roots to compensate for the amount of water they were putting out through their leaves. The result is rejection of some of their leaves.

The fact that the leaves are dropping early is a sign that the tree is healthy enough to try to protect itself. If the winter is reasonable and these trees get plenty of water this fall, they should leaf out next spring as if nothing ever happened.

. . .

Q:Could you please tell me more about a wild woody plant in a wooded area in front of my home? It looks like a large weed when it emerges from the ground, but if left alone it turns into a small tree. In July, it had clusters of white blooms, and by early August the blooms developed 1/8-inch diameter blue/black berries. Is this a weed or what?

Answer: I wasn't able to tell for sure from the photos you sent, so I forwarded them to Tom Wieboldt at the Virginia Tech Herbarium. Tom quickly replied that your tree is our common elderberry, Sambucus canadensis. They do grow wild in the woods in much of central Virginia. If you like the flowers, you may want to think of it as native plant rather than a weed.

. . .

Q:We would like to plant a low-maintenance, drought-tolerant "green carpet" and have read about native grasses, fescues and clovers being. We have acidic soil with lots of clay. Our lot was part of a tobacco farm 50 years ago. What is most appropriate for our area?

Answer: It would help to know in what part of Virginia you live and your sun exposure. In many areas of Virginia, the best native grass for a low-maintenance, drought-resistant lawn is common Bermuda grass (often called wiregrass). This is a perennial. It grows best in a mostly sunny yard. Since you mentioned your yard was a tobacco farm, I'll assume it does get lots of sun and is in south/central Virginia.

You can seed some clover into it in the fall. Clover is a legume, which means it can fix nitrogen in the soil and help fertilize the Bermuda. Fescues aren't considered native in Virginia. In fact, a statement I often make in lawn classes is, "If you did nothing to your lawn for a couple of years, fescue wouldn't show up and take over -- Bermuda and crabgrass would."

As for your acidic soil, even Bermuda will grow better in a pH of 6 to 6.5. Also, one good dose of fertilizer a year in late May or early June would help it choke out weeds.